This invention relates to the superconducting coil technology as well as the nuclear fusion reactor technology.
Superconductors making up superconducting coils for generating large amounts of magnetic field as in nuclear fusion reactors are of the cable-in-conduit (CIC) type. A CIC conductor comprises a large number of superconducting wires about 1 mm in diameter that are housed in a metal conduit. When a certain area of the conductor makes a transition to the normally conducting state, the area's temperature will arise due to Joule's heat and can damage the conductor. In order to prevent this problem, pure copper wires are also provided within the metal conduit. FIG. 1A is a cross section of a conventional CIC conductor in the form of a cable comprising about 200 superconducting wires 1 and 21 pure copper wires 2 of three kinds that are housed in a metal conduit 3. FIG. 1B is a cross section of a superconducting coil comprising a winding 6 housed in a coil case 8 as it consists of the CIC conductors 4 that are embedded in stainless steel plates with spiral grooves made in their surfaces.
The quantity of the pure copper wires depends on the decay time of the current flowing through the superconducting coil after the transition to the normal conducting state and the shorter the decay time, the smaller the quantity of heat generated and so is the amount of copper. The magnetic energy accumulated in the superconducting coil is consumed by a resistor provided outside of the coil.
One way to make the CIC conductor compact would be shortening the decay time so as to reduce the amount of pure copper wires and hence the size of the conductor. However, the toroidal field coil made of the superconducting coil for use in nuclear fusion reactors has an internal vacuum chamber for confining the hydrogen fuel. If the decay time of the superconducting coil is shortened, a large varying magnetic field is applied to the vacuum chamber and causes an induction current to flow into the vacuum chamber. The induction current combines with the magnetic field to create a large enough force to damage the vacuum chamber.